The present invention relates to a method for removal of sulfate groups and chlorate groups from an aqueous solution of alkaline metal chlorides.
When the aqueous solution of alkaline metal chlorides is used as the starting material for electrolysis, it is required to eliminate impurities contained in the raw material, and substances formed in the process of electrolysis. Especially when brine circulates through a closed system between a brine preparation process and an anode chamber as in electrolysis using an ion exchange process, it is known that a buildup of sulfate groups introduced from the raw material and chlorate groups formed in the anode chamber has an adverse influence on electrolysis performance, often resulting in quality drops of the resulting alkali hydroxides. It is thus inevitable to remove the sulfate groups and chlorate groups, etc. from brine.
On the other hand, chlorate groups are formed in the anode chamber during ordinary electrolysis using an ion exchange process, and then build up in brine. With the chlorate groups remaining unremoved, it is known that the concentration of chlorate ions in the brine increases, resulting in an electrolysis performance drop and a quality drop of sodium hydroxide. Thus, the chlorate groups, too, should be eliminated from the brine.
As a typical method of removing sulfate groups from brine, JP(A)60044056 discloses to remove sulfate groups from brine by means of a packed column system using a cation exchange resin containing a polymer type of hydrous zirconium hydroxide.
JP(A)03153522 discloses another method of removing sulfate groups from brine, wherein slurried zirconium hydroxide having a low water content is brought into contact with sulfate group-containing brine without being carried on a cation exchange resin to adsorb sulfate groups thereon, and the zirconium hydroxide with the sulfate groups adsorbed thereon is then transferred to another reaction vessel where it comes into contact with alkali in an aqueous solution.
Yet another method of removing sulfate groups from brine is disclosed in JP(A)60228691, wherein sulfate group-containing brine is diluted to adsorb sulfate groups on an anion exchange resin, and the anion exchange resin with the sulfate groups adsorbed thereon is then desorbed with concentrated brine, followed by crystallization and separation by cooling. This method is found to be undesirable in view of energy due to an increase in the number of steps involved.
With these sulfate group removal methods, however, it is impossible to remove sulfate groups and chlorate groups at the same time.
For chlorate group removal, a method of decomposing chlorate groups with the addition of hydrochloric acid to brine is known in the art. However, problems with this method are that much hydrochloric acid is needed and a reaction vessel for decomposing chlorate groups with hydrochloric acid has a reduced service life due to corrosion, etc. A further problem is that when chlorine dioxide formed during the decomposition of chlorates is mixed into chlorine, there is a risk of explosion, etc.
According to one approach to removing sulfate groups and chlorate groups from brine, a part of dilute brine is discharged out of a brine system and fresh salt is dissolved in the remaining brine to prepare brine for electrolysis. This approach is simple in operation, but a grave problem therewith is that a large amount of salt that is the raw material for electrolysis is rejected.
A method of fractional removal of both sulfate groups and chlorate groups by amphoteric ion exchange resin chromatography is also known in the art. Problems with the amphoteric ion exchange resin are, however, that its ability to remove them drops due to its premature degradation by poor chemical stability, and much chemicals must be used for desorption of the sulfate groups and chlorate groups upon absorption.
An object of the invention is to provide a method of separating and removing sulfate groups and chlorate groups at the same time from brine in the process of electrolysis of an aqueous solution of alkaline metal chlorides.